Study of energy transfer processes between the rare earth ions in NaGdF4 nanoparticles tri-doped with rare earth ions Yb3+, Er3+ and Ho3+ or Tm3+ was carried out. The luminescence spectra in short-wave infrared and visible ranges were investigated. It was shown that Er3+ → Ho3+ energy transfer leads to Ho3+ luminescence increase. Both Er3+ and Ho3+ luminescence peaks were observed in short-wave infrared range. For Er3+ and Tm3+ co-doped nanoparticles it was hard to separate luminescence peaks in SWIR. However, both Er3+ and Tm3+ luminescence peaks were observed in visible range. We attribute this effects to Tm3+ → Er3+ energy transfer which occurs due to overlap of Er3+ and Tm3+ luminescence bands in short-wave infrared range which leads to Tm3+ luminescence decrease. This hypothesis was confirmed by study of β-NaGdF4 tri-doped with Yb3+, Er3+ and Tm3+ luminescence spectra during heating. The intensity of Tm3+ luminescence increased during heating due to non-resonant nature of Yb3+ →Tm3+ energy transfer and the shape of spectra changed.
The Yb3+-Tm3+-doped NaGdF4 upconversion nanoparticles were studied as contactless nanothermometers for the first biological tissue transparency window under 980 nm excitation. The single hexagonal phase NaGdF4:Yb3+-Tm3+ nanoparticles were synthesized by solvothermal technique. The influence of dopants concentration and pumping power density on thermal sensitivity and temperature resolution of obtained nanoparticles was analyzed. It was shown, that an increase of Yb3+ doping concentration leads to a strong increase in near-infrared Tm3+ luminescence intensity, which corresponds to transitions 3F2-3H6, 3F3-3H6, 3H4-3H6 and could be used for thermometry. The measured efficiency of upconversion luminescence for 80% of Yb3+ and 2% of Tm3+ doped nanoparticles was 5.0% compared to 0.2% efficiency for 30% of Yb3+ and 0.5% of Tm3+ doped nanoparticles. Laser induced heating of synthesized nanoparticles with ratiometric temperature measurement was studied. The increase of pumping power density negatively affected the sensitivity, but increased the accuracy of measurement due to the increased near-infrared luminescence. In addition, the comparison of different wavelengths for ratiometric thermal calibration was performed. It was shown, that the use of 680-720 nm luminescence peak to 730-750 nm valley intensity ratio for thermometry promotes significant enhancement of thermal sensitivity and temperature resolution. Thermal sensitivity of 4%∙C-1 and temperature resolution of 0.6˚C in 30-36˚C region were obtained for NaGdF4 nanoparticles doped with 80% of Yb3+ and 2% of Tm3+.